Investigating Cr reduction in the Eastern Tropical North Pacific oxygen deficient zone with Cr isotope signatures

The global linear relationship between seawater chromium concentration [Cr] and its stable isotope ratio (log δ⁵³Cr) suggests preferential uptake/reduction of isotopically lighter Cr(VI) near the ocean surface followed by removal onto sinking particles, leaving the remaining dissolved Cr pool heavier.

Here, we report [Cr] and d⁵³Cr data on seawater samples from the Eastern Tropical North Pacific (ETNP) oxygen deficient zone (ODZ). Cr(III) was extracted by Mg(OH)₂ co-precipitation at sea on freshly filtered seawater and in the lab on thawed samples frozen at sea. The δ⁵³Cr(III) in the ODZ (120-500m) ranges from -0.14‰ to 0.31‰ and [Cr(III)] ranges from 0.85 to 2.16 nmol/kg, respectively, with the heaviest δ⁵³Cr(III) and highest [Cr(III)] concentration observed in the upper core of the ODZ (~175-250m) at station P2 (17°N, 107°W). Analysis of Cr(VI) remaining in the (filtered) supernatant from the Mg(OH)₂ Cr(III)-precipitations gives δ⁵³Cr(VI) as high as 2.98‰. These Cr isotope signatures are consistent with reduction and scavenging of isotopically light Cr. The depth where the most prominent Cr reduction was observed coincides with the secondary nitrite maximum, which implies the coupling of Cr reduction and microbial nitrogen cycling, especially denitrification. The fractionation factor of Cr reduction determined by calculated Cr(VI) isotope ratios assuming closed-system Rayleigh fractionation is -1.5‰. The bottom sample (450m) at a slope station (21.3°N, 105.7°W) has very positive δ⁵³Cr (1.85‰) but moderate [Cr] drawdown ([Cr]~2.34nmol/kg compared to 2.66nmol/kg at 400m). This sample does not fall on the global [Cr] vs. log δ⁵³Cr linear line, which might indicate a different Cr reduction mechanism from the others, possibly by Fe(II) reduced within organic-rich shelf sediments. Further incubation experiments will be conducted to investigate the Cr reduction by Fe(II) and denitrifiers and the associated Cr isotopic fractionation.